No, you don't. Motherboards come with built in sound, and most cases come with jacks as well. --- https://i.imgur.com/6sZC0hY.png "Some people have skeletons in their closets. I just have a bone in my pants." -Mitchell2003
Onboard sound output can only power self-powered speakers/headphones If you have speakers that does not plug into the wall you will need an amp/receiver as well --- WC i5 2500k @ 4.4 ghz, 2x GTX 670 DirectCU II, Samsung 830 128GB SSD, 16GB G.Skill 1600 DDR3, Zalman 1000W PSU, Cosmos 1000 Case, HP LP3065 @ 2560 x 1600 IPS
No you don't. If you have high impedance headphones (32 Ohms are about the least you'll see, so get those if you don't have an amp) or buy active speakers instead of passive (if you don't have a speaker amp) as onboard won't have much power.
Sound cards will have tiny amps in them that aren't very good. For the best audio quality you want an external DAC and AMP. --- Console war in a nutshell: http://imgur.com/xA6GJZ9.png